Oxidation is the most important process leading to engine oil degradation in internal combustion engines. The oil oxidation takes place primarily in the piston-cylinder area of the engine wherein a thin film of oil is subjected to high temperatures, high shear stress and combustion products. As a result, oil viscosity increases and acidic products, insolubles, sludge and varnish are formed. We have proposed a model for the oxidation of engine oils as it occurs in operating internal combustion engines in "Inhibition of Oxidation by ZDTP and Ashless Antioxidants in the Presence of Hydroperoxides at 160.degree. C.--Part I", M. D. Johnson, S. Korcek, M. Zinbo, SAE Technical Paper No. 831684 (1984), which is shown in FIG. 1. According to this model, the oxidation of engine oil is initiated by free radicals which are continuously produced during the combustion process or, additionally, may be derived from the decomposition of primary oxidation products, such as hydroperoxides, ROOH. These free radicals may react with the oil, RH, and, in the presence of oxygen, form peroxy radicals, RO.sub.2.sup..multidot.. In the absence of antioxidants, peroxy radicals can further react with additional oil to form hydroperoxides and alkyl radicals, R.sup..multidot.. This continues in a chain reaction process which can result in the formation of a high concentration of hydroperoxides. The chain reaction process can be inhibited by adding radical trapping antioxidants, AH, to the oil. In that case, peroxy radicals react preferentially with the radical trapping antioxidant to give non-radical products and keep the hydroperoxide concentration low. Formation of hydroperoxides is accompanied by radical formation from thermal and/or catalytic decomposition of the hydroperoxides which accelerates the oxidation. This initiation process can be prevented by the addition of peroxide decomposing antioxidants which convert hydroperoxides into non-radical products. Thus, to protect the oil against oxidative degradation in service, both types of antioxidants, radical trapping and peroxide decomposing, are used in engine oil formulations. Hindered phenols and amines are typical representatives of the first type of antioxidants, while zinc dialkylthiophosphates, ZDTP, are believed to react by both mechanisms. In some engine oils, ZDTP are the only antioxidants used, while in other oils they are supplemented by other antioxidants, such as amines and hindered phenols. In addition to the synthetic antioxidants added to the oil, engine oils may also contain "natural inhibitors" depending on the crude oil source and the method and degree of refining. Nevertheless, the majority of protection is provided by synthetic antioxidant additives. Determination of the antioxidant capability of engine oils is one of the most important technological parameters characterizing the oxidation properties of new oils and the remaining useful life of used oils.
Based on the above model for the oxidation of oils in internal combustion engines, it can be seen that hydroperoxide products are continuously formed in engine oils during engine operation due to a continuous influx of free radicals from the combustion process, even in the case when oxidation is inhibited by free radical trapping antioxidants. Since hydroperoxides at elevated temperatures initiate and accelerate further oxidation, their decomposition by peroxide decomposing antioxidants is not only a very important inhibition process but also an important process of antioxidant consumption. Therefore, in order to simulate oxidative conditions encountered in operating engines, the antioxidant capability of engine oils, base oils, and engine oil additives should be evaluated at elevated temperatures and under conditions of a continuous influx of free radicals or materials capable of breaking down into free radicals, e.g., peroxidic compounds.
Conventional laboratory tests currently used for the evaluation of antioxidant capabilities of engine oils, base oils and engine oil additives consist of determination of an inhibition period afforded by antioxidant species present in these materials. The inhibition period is determined from measurements of oxygen absorption (e.g., in an oxidation bomb apparatus), heat of reaction (e.g., DSC), formation of reaction products (e.g., acids, peroxides, insolubles, gaseous products), and/or change in physico-chemical properties (e.g., increase of viscosity). Exemplary of such tests are ASTM Standard Test Methods D943 and D2272 and that described in "Evaluation of Automotive Crankcase Lubricants by Differential Scanning Calorimetry", S.M. Hsu, A.L. Cummings, and D.B. Clark, SAE Technical Paper 821252, 1982. These tests are performed under various oxidative conditions at elevated temperatures (up to 160.degree. C.). None of these tests, however, include any provision to simulate the continuous influx of free radicals into the test materials. In the early stages of testing, the initiation of oxidation in these tests occurs only by the direct reactions of oxygen with oil components and it is only in the latter stages that the formation of free radicals is accelerated due to the radical decomposition of the hydroperoxides formed.
In the paper entitled "A Thin-Film Oxygen Uptake Test For The Evaluation of Crankcase Lubricants", C. Ku and S.M. Hsu, Lubrication Engineering, 40(2), 75-83 (1984), an oxidation test method is described which includes the addition of oxidized fuel components as a catalyst of oxidation. This test involves addition of the oxidized fuel components prior to beginning the oxidation test and, therefore, does not simulate the continuous interaction of combustion derived free radical species with the oil, which results in the continuous formation of hydroperoxides.
Another type of test which is used for evaluation of antioxidant capacity of engine oils or determination of antioxidant concentration in various hydrocarbonaceous materials is described in U.S. Pat. No. 4,155,713 to Mahoney. This type of test comprises the determination of free radical trapping antioxidant capability by titration with peroxy radicals at low temperatures (usually 60.degree. C.). This titration with peroxy radicals, simulates a continuous influx of free radicals, however, it can be performed only at reaction temperatures which are much lower than those encountered by oil in an operating engine. Under such test conditions, initiation of oxidation by the free radicals produced by the thermal decomposition of hydroperoxides is completely suppressed and interactions of antioxidants with hydroperoxides are partially suppressed.
Thus, currently used laboratory tests for evaluation of antioxidant capabilities of engine oils, base oils and engine oil additives do not simulate the oxidative conditions encountered in an operating internal combustion engine as described by our model.